1. Field of the Invention
The present invention relates to mechanical positioning devices and, in particular, to mechanical positioning devices for positioning a workpiece in a work space, which devices are operable from a position which is remote to a work space which may be enclosed from or inaccessible to the remote position.
2. Prior Art
In numerous industrial processes, scientific research and instrumentation applications it is necessary to introduce into, or to move specimens or work-in-process, within a controlled atmosphere which is maintained inside a chamber. In many processes the controlled atmosphere is frequently at reduced atmospheric pressure, ranging from partial atmospheres to hard vacuums. The materials to be treated in such processes are placed in a vacuum chamber in which there are various source materials available for such processes as plasma etching, sputtering, ion implantation, chemical vapor deposition, and similar processes.
In research instrumentation applications, the vacuum environment may be the interior of, for example, an electron microscope, a mass spectrometer, or the like.
Where the process requires materials to be introduced into or removed from a target area within a chamber, it is necessary either that the chamber have an access port which can be readily opened for movement of the materials, or some means of manipulating the material within the chamber from the exterior of the chamber. Other difficulties such as contamination aside, if the chamber must be opened to the surrounding atmosphere each time that a specimen is moved to a different position, the overall process time is increased to unacceptable levels. It is a time consuming and energy-wasting process to allow the interior of the chamber to stabilize to ambient pressure, and then to re-establish the level of pressure which is necessary for the continuation of the process, each time that material is introduced into or taken out of the chamber.
Furthermore, the intrusion of ambient atmosphere into the controlled atmosphere of the process apparatus or instrument which causes the introduction of contaminants or a change in the pressure cannot be ignored. While in some applications it is merely desirable to maintain the vacuum or atmosphere of the process apparatus, or the interior of the scientific instrument, free from intrusion of impurities in the ambient atmosphere into the interior of the apparatus, for some applications, of course, a high vacuum environment is fundamental to operations and maintaining the vacuum at all times is an overriding requirement.
As a practical matter, these considerations dictate that for almost all vacuum processes the vacuum must be maintained throughout the process. To avoid this time and energy wastage, the process apparatuses and instruments are almost invariably equipped with some form of transport device which allows movement of materials from one position to another in the chamber without necessitating the breaking of the chamber seal.
Devices used for sample transferring and positioning and similar functions within the vacuum chamber ideally will have maximum flexibility of movement. Changing from one sputtering target to another, for example, can require a combination of both linear and rotary motions. By providing for sufficiently flexible mechanical movement inside the vacuum chamber, operable from outside of the chamber, the need to break the vacuum and release it to atmospheric pressure is reduced. Generally for those chambers equipped with a flexible mechanical manipulator, only one pumpdown of the vacuum chamber will be required for a given process, if the mechanical motion is well-planned, and if all needed facilities are provided inside the vacuum chamber.
In the past, various forms of mechanical manipulators have been proposed for the solution to providing high vacuum deposition chambers, and the like, with satisfactory mechanical positioning devices. These solutions have taken the form of several separate classes of devices such as:
a) Rack and pinion mechanisms. These devices provide reliable linear motion, but have limited rotary motion.
b) Bellows Assemblies. This solution provides linear or rotary motion, but bellows are expensive, can fail, and require extremely strong structural supports to withstand atmospheric pressure, thereby obscuring desired tactile feedback.
c) Conventional Magnetically Operated Manipulators. These devices use a single shaft to provide translation and rotation with one coupled motion--the shaft moves in and out and rotates. This linkage between rotary and linear movements, however, severely limits motion flexibility.
Conventional magnetically operated manipulators employ a cylindrical housing attached to the side of the chamber, and protruding radially from the side of the chamber, within which is located a moveable rod.
In general, to accomplish the movement of the rod from a position outside of, e.g., a vacuum chamber, a magnet carriage slides over the exterior of a cylindrical housing, couples a magnetic field through the housing to a magnet follower which runs inside the housing and is attached to the manipulator rod. If the magnetic field is coupled to the follower asymmetrically, so that a field vector perpendicular to both the translational and the rotational direction is produced, then it is possible to exert both rotational and translational forces on the magnet follower, and therefore on the rod which is coupled to the magnet follower.
While the principles of operation stated above are straightforward, the successful implementation of the principles have posed problems of design which have heretofore not been satisfactorily solved.
As noted above, in the past, a single rod has been used for the purpose of providing both translation and rotation. In order to allow both translational and rotational movement of the rods, the rod had to be mounted in, typically, sleeve-type bearings, or slidable roller bearings such as Thompson linear bearings.RTM.. These bearings allow both rotational and translational movement, but at the expense of much higher stiction and friction than ball or roller bearings used in an optimized configuration will allow. This is particularly true when the bearing is used inside a chamber in which the bearing must be operated in a dry condition. A particularly severe limitation of bearings which must support both linear and rotary motion is that the bearing may be easily cross-loaded, rendering it either temporarily inoperative due to mechanical binding or, in the worst case destroy the bearing or rod. Cross-loading is particularly likely when high loads are placed on the bearing, as when moderate to heavy loads are supported at the maximum extension of the manipulator rod.
Many prior art conventional magnetic manipulators also have had relatively low coupling forces between their external magnets and internal followers due to the absence of optimized magnets and design. This results in a highly elastic coupling, which in turn results in the position of the actuators does not corresponding to the position of the manipulators. This inherent sloppiness limits the precision with which motion can be transmitted by such manipulators, and, in turn, limits their usefulness in many situations.
The problems inherent in the designs of earlier precision magnetic manipulators have been primarily this lack of precision and the lack of good tactile feedback as a result of the stiction and friction forces which occur in the housing. The forces which produce good tactile feedback, essentially the transmission of the forces experienced by the movement of the manipulator under its load are obscured by the forces produced by the manipulator itself due to its inherent stiction and friction.
These problems have lead to the widespread disenchantment with precision magnetic manipulators of the older designs, and gradual movement toward different types of manipulators altogether.